By examining decades' worth of stored bacteria samples, researchers have determined how a benign organism evolved into a deadly pathogen that causes necrotizing fasciitis, commonly known as flesh-eating bacteria disease.

Using genetic sequences from more than 3,600 strains of bacteria, scientists were able to see that it took only four steps to create the unusual microbe that spreads rapidly and destroys the body's soft tissue. Their report was published Monday in the Proceedings of the National Academy of Sciences.

Necrotizing fasciitis is caused by several types of bacteria, most commonly group A Streptococcus. (See images of Streptococcus and other microbes in the "Small, Small World" photo gallery.) An international group of researchers sequenced the genomes of group A